A conventional solution for a slave mobile phone to join a pre-established call between a master mobile phone and a remote communication apparatus requires the use of additional services such as conference calls, said services being provided by a Mobile Network Operator. All Mobile Network Operators do not support this service. Most of the time, this service requires a subscription, and additional fee.
Such a solution requires that the master mobile phone which wants to enable the conference call:                suspends the pre-established call,        gets the phone number of the joining slave mobile phone,        dials the slave mobile phone (assuming that this feature is supported by the master mobile phone and the Mobile Network Operator),        waits until a second call between the master and slave mobile phones is established,        links the two calls by a mechanism provided by the Mobile Network Operator.        
A similar situation may occur when the master mobile phone user takes a picture and wants to send it. The following steps need to be carried out:                loading/activating the transfer application (pressing several keys, going through menu to do so) on the master mobile phone,        making the slave mobile phone in a “receptive state for receiving the picture (also by pressing keys and going through menu),        establishing a communication link between the master and slave mobile phones, and        once the communication has been established, sending the picture, for example by pressing a key.        
The above-described solutions are not satisfactory and extremely complex from a user point of view, since they require a major involvement from the user. According to the example of the conference call, establishing a second call is already a challenge on most of the mobile phones today, and, in addition to this complexity, the user must know how to activate the conference call service of the Mobile Network Operator. As a matter of fact, whereas mobile phone users are familiar with the basics functions provided by the mobile phones (e.g. dialing up, answering), they are extremely unfamiliar with advanced functions such as call forwarding or conferencing.
These solutions are also expensive. Indeed, the additional second call will be charged to the caller's account. Such a solution is acceptable when the master and slave mobile phones are not collocated. It is not acceptable when the two mobile phones are physically nearby.